244
MODIFICATION OF THE GREAT LAKES
peated, and the difference found to be 18.470 feet, the point at
Sacketts Harbor having gone up, as compared to the point at
Charlotte, 0.061 foot, or about three-fourths of an inch. Similarly
it was found that between 1858 and 1895 a point in Port Colborne,
at the head of the Welland canal, as compared to a point in
Cleveland, Ohio, rose 0.239 foot, or nearly three inches. Between
1876 and 1896 a point at Port Austin, Michigan, on the shore of
Lake Huron, as compared to a point in Milwaukee, on the shore
of Lake Michigan, rose 0.137 foot, or one and one-half inches;
and in the same period a point in Escanaba, at the north end of
Lake Michigan, as compared to the same point in Milwaukee,
rose 0.161 foot, or about two inches.
There is no one of these determinations that is free from doubt;
buildings and other structures on which the benches were markt
may have settled, mistakes may have been made in the earlier
leveling, when there was no thought of subjecting the results to
so delicate a test, and there are various other possible sources of
error to which no checks can be applied; but the fact that all
the measurements indicate tilting in the direction predicted by
theory inspires confidence in their verdict. This confidence is
materially strengthened when the numerical results are reduced
to a common unit and compared.
Summary of Distances, Time Intervals, and Measurements of Differential
Earth Movements
Pairs of stations.
%
J
J|J
Miles. Miles.
Years.
Feet.
Feet.
Feet.
Sackettsarbor
86
76
22
. 061
.37
.18
and Charlotte...
Port Colborne and )
7
PoCleveland....... 158
141
37
. 239
.46
.11
Port Austin and10
SMilwukee......and 259
176
20
. 137
.39
.09
SMilwaukee .......
Escanaba and Mil- 192
186
20
. 161
.43
.06
Mean......
.41
Weighted mean......
.42
-
.05